^4^ GEOLOGY. 



Unless new intrusions of lava occur, or unless heat is otherwise 

 renewed at the proper points, it is probable that all existing geysers 

 will become extinct within a time which is, geologically, short. New 

 geyser regions ma}^, however, develop as old ones disappear. 



Artesian wells. — Originally the terms artesian wells and flowing 

 wells were synonymous; but at the present time any notably deep 

 well is called artesian, especially if it descends to considerable depths 

 below the mantle rock. The artesian well which does not flow, does 

 not differ from common wells in principle ; but being deeper, the water 

 which it affords is often more thoroughly filtered and frequently more 

 highly mineralized than that of other wells. The flowing well is really 

 a gushing spring, the opening of which was made by man. 



Flowing wells ^ depend upon certain relations of rock structure, 

 water supply, and elevation. Generally speaking a flowing well is 

 possible in any place underlain by any considerable bed of porous rock, 

 if such rock outcrops at a sufficiently higher level in a region of ade- 

 quate rainfall, and is covered by a layer or bed of impervious, or rela- 

 tively impervious rock. This statement involves four conditions, 

 all of which are illustrated by Fig. 199, where a is the bed of porous 

 rock. It is not necessary that the beds of rock form a structural basin, 

 nor is it usually necessary to take account of the character of the rock 

 beneath the porous bed which contains the water. 



The bed of porous rock is the ^'reservoir" of the flowing wefl. For- 

 mations of sand or sandstone, and of gravel or conglomerate, most 

 commonly serve as the reservoirs. In order that it may contain abun- 

 dant water it must have some thickness, and its outcropping edge 

 must be so situated that the water may enter freely and be replenished, 

 chiefly by rain, as the water flows out at the well. 



A relatively impervious layer of rock above the reservoir (b, Fig. 199) 

 is most important ; otherwise the water in the reservoir will leak out, and 

 there will be little or no '^ head " at the well site. Thus if the rock over- 

 lying stratum a (Fig. 199) were badly broken, the fractures extending 

 up to the surface, the conditions would be unfavorable for flowing wells. 

 Under such conditions, wells in the positions of those shown in Fig. 199 

 might get abundant water, but they would not be likely to flow. If the 



1 Chamberlin. Geol. of Wis., Vol. I, pp. 689-97, and Fifth Ann. Rept., U. S. Geol. 

 Surv., pp. 131-73. The former a brief, and the latter an elaborate, exposition of 

 the principles involved. 



